The materials of the present invention are most importantly substrate candidates for electronic devices. Several processes in the manufacture of electronic devices such as liquid crystal displays (LCDs), solar cells, electronics, microelectronics etc. include steps that are performed at extremely high temperatures. For example, active matrix LCDs employ an active device such as a diode or thin film transistor at each pixel thereby enabling high contrast and high response speed. Although many display devices currently utilize amorphous silicon (a-Si), the processing of which may be accomplished at temperatures under 450° C., polycrystalline-silicon (poly-Si) processing is preferred. Poly-Si has a much higher drive current and electron mobility thereby increasing the response time of the pixels. Further, it is possible, using poly-Si processing, to build the display drive circuitry directly oh the glass substrate. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques. The most efficient poly-Si processing methods operate at temperatures of at least 800° C., such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. This fabrication process typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in the substrate being heated to temperatures in the range of 800° C. Fused silica has a sufficiently high strain point of 990-1000° C., but its thermal expansion coefficient (C.T.E.) is significantly lower than that of silicon (C.T.E. 5×10−7/° C. v. C.T.E. 37×10−7/° C.).
For other electronic devices, common, processing steps also require high temperature substrates to withstand processing. Most high level electronic fabrication requires annealing of the gate oxide and dopant activation. These processes occur at temperatures in excess of 800° C.
Even in the case of single crystal silicon (x-Si) fabrication techniques that employ a thin layer of single crystal silicon bonded to a substrate, high temperature substrates are required. Single, crystal silicon allows for even greater electron mobility than that achieved with poly-Si. The bonding step often requires high temperatures as well as the gate oxide and dopant activation steps previously described.
A need exists, then, for a glass that (1) has a high strain point (>800° C.), (2) does not require costly heat treatments after fabrication, (3) has a CTE match close to that of silicon, and (4) can be melted in a conventional melting unit. In addition, the glass will preferably be transparent to visible radiation and be chemically durable. These several qualities are needed in glasses for production of such varied products as flat panel displays, photovoltaic cells, photomasks, optomagnetic disks and tubing and fiber applications that require stability at high temperatures.
Flat panel displays employ sheet glass that necessarily is transparent at visible wavelengths as well as into the ultra violet. It is also necessary that the glass sheet be adapted to production of a silicon layer on the glass surface. Initially, the silicon layer applied was amorphous silicon (a-Si). Fabrication of such devices required temperatures no greater than 350° C. Suitable glasses were readily available for use under these conditions.
The evolution from a-Si to poly-Si and x-Si as a coating material has presented a major challenge to use of a glass substrate. Poly-Si and x-Si coatings require much higher processing temperatures, in the range of 600-1000° C.
A primary purpose of the present invention is to provide a glass that has properties suited to production of a poly-Si or x-Si coating on its surface.
Another purpose is to produce a glass having a sufficiently high strain point to permit processing at 800-900° C.
A further purpose is to provide a glass that can be melted by conventional procedures, and that can provide a substrate for application of a high quality, poly-Si or x-Si film.
A still further purpose is to provide an electronic device, in particular, a flat panel display, and having a high-quality, poly-Si or x-Si, thin film on its surface.
Another purpose is to provide a novel glass family consisting essentially of Al2O3—P2O5—SiO2, and optionally containing selected oxides such as alkali, alkaline earth, transition metal oxides, as well as oxides of the lanthanide series.